Vol. 60.] IGNEOUS EOCKS OF PONTESFOKD HILL. 475 



extinction-angles np to 15°. The structure is typically pilotaxitic, 

 though in places, where there is more residual glass, it might be 

 more exactly termed • hyalopilitic' 



These rocks, which are evidently of the andesite-series, though 

 perhaps originally more acid than those in the same series farther 

 north, pass at once into typical acid tuffs or breccias, 

 which culminate in the South-Eastern Rhyolite. 



A hard, flaggy, pink, felsitic-looking rock with green angular 

 chips (-538, 540, 54-3, 545) is the first band of these markedly-acid 

 tuffs, with structures generally like those in the Westphalian 

 Devonian tuffs described by Miigge. 1 



Xo. 538 is a rather fine-grained variety, made up of very small 

 fragments of red, vesicular, altered glass with the typical ' Bogen- 

 struktur,' set in a dirty-green matrix of fine glassy and crystal 

 dust. Larger crystals of felspar, showing both simple and lamellar 

 twinning, are present, together with rounded lapilli of vesicular 

 glass measuring up to 0'14 inch across ; in one place the vesicles 

 have been drawn out into long and extremely-fine tubes. The 

 rock has the same general structural character as the palagonite- 

 tuffs, and was evidently formed under much the same conditions, 

 in this case by the breaking up of a perlitic and very vesicular 

 acid glass, the glass becoming afterwards strongly coloured with 

 iron-oxide. Its percentage of silica is 74*83, and the specific gravity 

 is 2-64. 



Xo. 540 is in the same band as 538, but is coarser in texture, 

 with green chips measuring up to - 3 inch in length. The matrix, 

 greenish in colour, is made up of fine glassy dust, embedded in 

 which are many minute red splinters of glass, with curved edges, 

 and often showing the optical phenomena of tension, like those 

 observed in Rupert's drops ; together with phenocrysts of orthoclase- 

 felspar (some 0*03 inch long), and irregularly-shaped lapilli of 

 green, fine-grained, banded tuff. 



This glassy breccia is followed by a bright-red and green flaggy 

 grit (539, 541), in places dipping north-westward into the hill 

 at about 80°. A specimen (539) is a very striking rock under the 

 microscope (PI. XLIII, fig. 4). It is made up of lapilli measuring 

 about 0*U2 inch across, mostly of green and brownish-red vesicular 

 glass, often showing what looks like perlitic structure, but may be 

 the vesicular structure previously described in the matrix of the 

 Xorthern Rhyolite (pp. 457. 458) ; others are fragments of dark- 

 brown, nearly black glass, crowded with felspar-microlites ; others 

 again of felspar-crystals, more or less broken, usually exhibiting 

 simple twinning, together with occasional rounded blebs of quartz 

 0*05 inch across. The rock bears a general structural resemblance 

 to the grit, in the Andesite-Group at the top of the galley (512), 

 but the fragments are more glassy and the rock as a whole more 

 acid. The bright-red colour of this rock is due to the large amount 

 of haematite that has developed in the glass. 



1 ' Untersuchungen iiber die Lenneporphyre in Westfalen & den angreuz- 

 enden Gebieten ' Neues Jahrb. Beilage-BandViii (1893) p. 542. 



